Studying the movement patterns of small flying insects, such as pollinators, is a major challenge in biology. Here we introduce an original approach to track the 3D motion of flying insects using a millimeter-wave radar imaging system. 3D images are obtained from the beam scanning of a Frequency-Modulated Continuous-Wave and Single-Input Multiple-Output radar operating at 77 GHz. We derive the flight trajectory of insects and mitigate the electromagnetic clutter from the 3D radar images. To illustrate our approach, 3D flights of isolated bumblebees are radar tracked in a limited volume during 5.2 minutes with a time resolution of 60ms. The radar tracks are compared with video recordings for validation purpose. The tracking volume of an insect using a single radar is estimated at 5.3m 3 , and we show that the volume can be tripled when three radar imaging systems are deployed. Finally, we discuss how our new radar-based technique can be extended to track a broad diversity of small flying animals at different spatial scales and simultaneously in the lab and in the field.